1. Field
The following invention disclosure is generally concerned with semiconductor based light sources and specifically concerned with semiconductor based broadband light sources having high output flux.
2. Prior Art
Practitioners of the optical sciences will be quick to point out several techniques for simulating a white-light, light emitting diode LED. ‘Simulate’ is specified as the intrinsic properties of LEDs demand that they emit light in relatively narrow bands; white light is by definition, a broad optical spectral band. To date, there are no real ‘broadband’ LED emitters which truly produce white light at the diode junction in sufficient quantity and efficiency so as to be commercially viable. Rather, there are several configurations deployed to mix light from a plurality of narrow band individual sources.
For example, one might combine in close proximity red, green, and blue emitting diode chips. If the associated brightness of each is appropriate, and the system is viewed from sufficiently far away whereby the eye does not resolve the individual chips, it will appear to be a ‘white’ LED light source. Many difficulties are found in such systems and these are currently not in favor. They are problematic in manufacturing and their performance is not favorable.
In some systems, a single diode chip which produces ultraviolet light is combined with three different phosphors which emit light in various parts of the spectrum. The ultraviolet chip stimulates each of the phosphors and their emissions combine to form white light. These systems have yet to win favor for their lack of efficiency.
A very useful alternative which has recently become enabled via high brightness blue emitting diodes is realized in the following manner. A high brightness blue LED is placed on a substrate. A coating or slurry of phosphor is applied thereon the top of the semiconductor chip. This special phosphor is stimulated by blue light emitted by the chip. When stimulated, the phosphor emits light, albeit with less energy (longer wavelength) than the stimulating light. Phosphors which are stimulated by blue light and emit yellow light have been used to form ‘White’ LEDs. It is tricky to get the coating of phosphor just right. The interaction cross section dictates how much of the blue light is converted to yellow. As it is desirable to have just the right amount of blue light mix with just the right amount of yellow light, the thickness and density of the phosphor coating has a great effect on the interaction cross section. The nature of the phosphor grain also effects the interaction cross section and scattering properties. In particular, the size and shape of the phosphor particles changes the interaction characteristics. Because geometries particular to semiconductor chips and LED device packaging, commonly used techniques present problems in angular uniformity, among others.
For example, such configurations typically employ a blue emitting LED with a wavelength of about 455 nm and a yellow emitting phosphor such as cerium doped YAG, yttrium-aluminum-garnet, having its peak secondary emission at about 560 nm the half-width of the spectrum, that equals about 120 nm. This results in a color temperature of about 8000° K. and a low CRI of about 70.
U.S. Pat. No. 5,998,925 describes systems where a YAG based phosphor is used to convert blue light emitted from a nitride semiconductor into yellow light.
Shimizu presents similar invention in his U.S. Pat. No. 5,998,925, which we consider as an analogue. According to this patent, for semiconductor structures of InGaN, it is suggested using photophosphor out of alluminium-yttrium garnet in accordance with the formula:Y3-x-yGdxCe3(Al,Ga)5O12.
Combining such photophosphor with light from a semiconductor, i.e. yellow light at approximately γ=560 nm, allows one to achieve a combined output radiation of a white nature or close to white color with various color tints (bluish, yellowish etc.) This construction became widely used in manufacturing, though it is not devoid of deficiencies including at least:
Relatively low color rendering, defined in the form of color index Ra≦70 units;
Insufficiently high optical emission output out of aluminum-yttrium garnet (photophosphor) due to a large difference in refraction indices of phosphor grains (n=1.95) and organic polymer (n=1.45) used as glue for fixing grains to emitting facets of a light-emitting diode;
High cost of phosphor conditioned by using expensive rare-earth metals such as yttrium, gadolinium, cerium at the phosphor synthesis.
All the mentioned deficiencies led to creating a new photophosphor for light-emitting diodes, the base of which are strontium orthosilicates with a general formula:Sr2-xEuxSiO4.
Orthosilicate photophosphor emits in green or green-yellowish areas of visible spectrum (from γmax=520 nm up to γmax=550 nm) with half-width of radiation spectrum equal γ0.5=80 nm±20 nm. It is expected that orthosilicate photophosphors will compete with standard aluminum-yttrium materials.
Particular attention is drawn to US patent application publication numbered 2004/0251809, which discloses a phosphor and light emitting device using same phosphor. In particular, a phosphor comprising a host material composed of a compound having a garnet crystal structure represented by the general formula:M1aM2bM3cOd 
Wherein M1 is a die feeling metal elements, M2 is a trivalent metal element, M3 is a tetravalent metal element containing at least Si, ‘a’ is between 2.7 to 3.3, ‘b’ is 1.8 to 2.2, and ‘c’ is between 2.7 and 3.3, and ‘d’ is a number 11.0-13.0. It is particularly important to note that this a material is based upon the garnet crystal structure. In addition, the absence of halogens is notable.
Inventors Tasch, et al teaching U.S. Pat. No. 6,809,347 issued Oct. 26, 2004 luminophore which comes from the group of alkaline earth orthosilicates and which absorbs a portion of light emitted by a light source and emits light in another spectral region. These alkaline earth orthosilicate photophosphors are activated with bivalent europium. To improve the broadband nature of these systems, additional luminophore selected from the group of alkaline earth aluminates activated with bivalent europium and/or manganese, and additional luminophore of a red-emitting type selected from the group Y(V,P, Si)O4:Eu or can contain up claim earth magnesium disilicate.
Yet another white light system is presented by Taiwanese company Vtera Technology Inc. in U.S. Pat. No. 6,825,498. In this system a ‘P’-type ZnTe layer or ZnSe layer is formed along with the LED. Blue light from the LED is absorbed by the ZnTe or ZnSe layer and converted in wavelength to a yellow green light. In this manner, a wavelength conversion layer is provided in conjunction with a typical blue emitting LED.
Inventors Ellen's et al, present in their disclosure, U.S. Pat. No. 6,759,804 issued Jul. 6, 2004 illumination devices with at least one LED as a light source. Wavelength conversion is achieved by way of a phosphor which originates from the class of (Eu, Mn)-coactivated halophosphates, where the cation and is one of the metals Sr, Ca, Ba.
The same inventors further teach in their U.S. Pat. No. 6,674,233 further inventions relating to illumination units having an LED as a light source. However these systems include phosphors from the class of cerium activated sialons, the sialon corresponding to the formula:Mp/2Si12-p-qAlp+qOqN16-q:Ce3+
U.S. Pat. No. 6,501,100 is entitled: “White light emitting phosphor blend for LED devices”. There is provided a white light illumination system including a radiation source, a first luminescent material having a peak emission wavelength of about 570 to about 620 nm, and a second luminescent material having a peak emission wavelength of about 480 to about 500 nm, which is different from the first luminescent material. The LED may be a UV LED and the luminescent materials may be a blend of two phosphors. The first phosphor may be an orange emitting Eu2+, Mn2+ doped strontium pyrophosphate, (Sr0.8Eu0.1Mn0.1)2P2O7. The second phosphor may be a blue-green emitting Eu2+ doped SAE, (Sr0.90-0.99Eu0.0-0.1)4Al14O25. A human observer perceives the combination of the orange and the blue-green phosphor emissions as white light.
In U.S. Pat. No. 6,577,073 an LED lamp includes blue and red LEDs and a phosphor. The blue LED produces an emission at a wavelength falling within a blue wavelength range. The red LED produces an emission at a wavelength falling within a red wavelength range. The phosphor is photoexcited by the emission of the blue LED to exhibit a luminescence having an emission spectrum in an intermediate wavelength range between the blue and red wavelength ranges.
U.S. Pat. No. 6,621,211 presents white light emitting phosphor blends for LED devices. There is provided white light illumination system including a radiation source, a first luminescent material having a peak emission wavelength of about 575 to about 620 nm, a second luminescent material having a peak emission wavelength of about 495 to about 550 nm, which is different from the first luminescent material and a third luminescent material having a peak emission wavelength of about 420 to about 480 nm, which is different from the first and second luminescent materials. The LED may be a UV LED and the luminescent materials may be a blend of three or four phosphors. The first phosphor may be an orange emitting Eu2+, M+ activated strontium pyrophosphate, Sr2P2O7:Eu2+, Mn2+. The second phosphor may be a blue-green emitting Eu2+ activated barium silicate, (Ba,Sr,Ca)2SiO4:Eu2+. The third phosphor may be a blue emitting SECA phosphor, (Sr,Ba,Ca)5PO4)3CI:Eu2+. Optionally, the fourth phosphor may be a red emitting Mn4+ activated magnesium fluorogermanate, 3.5 MgO 0.5 MgF2 GeO2:Mn4+. A human observer perceives the combination of the orange, blue-green, blue and/or red phosphor emissions as white light.
While systems and inventions of the art are designed to achieve particular goals and objectives, some of those being no less than remarkable, these inventions have limitations which prevent their use in new ways now possible. Inventions of the art are not used and cannot be used to realize the advantages and objectives of the inventions taught herefollowing.